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How do I prepare a DXF file for imort as a problem gemetry?

Last Updated: Oct 30, 2013 04:12PM GMT

Question

Importing a dxf file into LimitState:GEO does not produce a model but I can open the file in other programs. Are there any restrictions on what I can import?

Solution

Yes - there are restrictions on the object types that you can import in a DXF:

Permitted DXF commands

The following commands can be used to create a DXF file that is readable by LimitState:GEO (other commands, including those used to generate 3D objects, will not be recognized):

  •  Point
  •  LWPolyline
  •  Line
  •  Polyline
  • XLine
  •  Spline
  •  Vertex

Closed loops of lines forming ‘islands’ within other closed loops will be recognized and imported without difficulty. However, additional lines will be automatically generated to connect these ‘islands’ to the surrounding geometry (to allow LimitState:GEO to analyse the problem properly). It is a good idea to 'explode' the drawing just before saving.

Ensure loops are properly closed

In most CAD packages it is possible to work to a very fine precision. This can sometimes lead to situations where lines that appear to intersect do not actually meet in reality. Whilst not necessarily causing a problem in the CAD model, if two lines of a loop do not intersect when importing a DXF into LimitState:GEO, a solid object will not be formed. Care should therefore be taken to ensure that, where objects are expected to be converted to solids, these are properly drawn in the CAD model before exporting to DXF (working with a grid or object snap turned on can help with this).

Remove unnecessary objects

Only those types of drawing object listed above will be recognized and imported into LimitState:GEO. If a CAD file is being worked on specifically for import into  LimitState:GEO then text, hatching and other similar objects should be removed before the DXF file is generated. Failure to do this may lead to unwanted objects being created or to a loss of program efficiency as LimitState:GEO processes the superfluous data. It is also advisable to check that there are no completely overlapping (duplicate) lines in the file.

Keep the drawing as simple as possible

When importing from a DXF file, every intersection of two or more lines will be converted to a LimitState:GEO vertex. These vertices are, in turn, regarded as nodes along the boundaries of solid elements at the time of analysis. As LimitState:GEO normally works with a fixed number of nodes, excessive detail (leading to large numbers of boundary nodes) will reduce the number of nodes inside solid elements and potentially reduce the accuracy of the solution obtained. Wherever possible, the user is advised to appropriately simplify the ground profile of any problem before importing and try to achieve a suitable balance of boundary and internal nodes in the LimitState:GEO model. This advice is particularly relevant when working with DXF files containing multiple layers.

Work at the correct scale

LimitState:GEO is set to work in either metres or feet (dependent upon the choice of units in the Preferences menu). It should therefore be ensured that all imported DXF files are drawn to an appropriate scale. It is best to set an appropriate point (normally the bottom left corner of the domain or nearby) to (0,0).

Be aware that layers are flattened

Since LimitState:GEO works with planar models, when multi-layered DXF files are imported, all layers are first flattened. Any overlapping lines that were distinct in the original file will no longer be separate, which may result in undesired problem geometries. It is recommended that such DXF files are simplified to a single layer representing the problem the user wishes to model.

Work in the xy plane

Care should be taken to ensure that the correct viewpoint is being used when preparing a DXF file for import. LimitState:GEO works in the xy plane, which corresponds to the ‘Plan’ or ‘Top’ 3D viewpoint in most CAD packages. Objects that lie predominantly in the xz or yz planes may be imported in a way that is not desirable. It is recommended that DXF files containing 3D components are simplified to a single (2D) layer representing the problem the user wishes to model.

Detailed guidance on importing DXF geometries is given in the LimitState:GEO User Manual.

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